1. Field of the Invention
This invention relates to a magnetic recording device and particularly to a device for the recovery of recorded data from a magnetic recording medium having a relative movement with respect to a sensing transducer. This invention relates more particularly to a novel and improved read back compensation circuit for a magnetic recording device such as a floppy disk drive.
2. Description of the Prior Art
Magnetic recording devices capable of storing digital data, especially binary data, are used with electronic data processing systems, communications systems, and the like. To improve the performance capability of such magnetic recording devices, there has been continuing effort to improve the recording density of such devices. That is, the number of bits or digits of digital data recordable on an inch of the magnetic recording medium is desired to be as large as possible. There has also been continuing effort in the faithful reproduction of the recorded digital data. Playback or readback of a magnetic recording is usually accomplished by amplifying the small amplitude signals induced in a magnetic reproducing or read head as the magnetic medium moves past the latter. Operational difficulties relating to faithfully reproducing the recorded data increase as the reading speed increases.
On the magnetic medium the digital data are represented either as changes in magnetic flux, as the polarity of magnetic flux, or as different frequencies of changing magnetic flux in a given area of the medium, which is often referred to as a "bit cell" or "cell." For example, a positive magnetic remanence may represent a binary "1", while a negative magnetic remanence may represent a binary "0". Or, for example, in a frequency modulation or bifrequency recording, a binary "0" may be represented by a first frequency of flux reversals, and a binary "1" may be represented by a second frequency of flux reversals. That is, for instance, a magnetic flux waveform having an "F" signal frequency may represent a binary "0", whereas a magnetic waveform having a 2F signal frequency may represent a binary "1". Frequency modulation is often used in floppy disk drives, where either a single-density frequency modulation encoding scheme (FM) or a double-density encoding scheme (MFM) is applied, see article "A Method of High Density Recording on Flexible Magnetic Discs" in the magazine "Computer Design", October 1976, pages 106 to 109.
A floppy disk drive is a random access storage device which uses a removable "floppy disk" as the storage medium. A single disk will presently store approximatley 3 to 6 megabits of data. The "floppy disk " is a magnetic-oxide-coated disk of some flexible material. It is usually sealed within a plastic envelope for protection, thus forming a disk cartridge. The disk cartridge should be handled and stored in an environment which is free from magnetic influences.
In floppy disk drives as well as in other magnetic recording devices, operational difficulties in the faithful reproduction of recorded digital signals include so-called peak shifting. Peak shifting is a term applied to the readback signal, which indicates a shift of recording cell boundaries; peak shift also looks like a longer duration recorded pulse or signal than was acutally recorded (see for instance U.S. Pat. No. 3,597,751).
In other words: For alternate "1's" and "0's" there should be a sharp demarcation between adjacent recorded areas on the recording medium. This should also be true for groups of "1" cells alternating with one or more "0" cells. However, a comparison between the recording signal and the readback signal reveals that the two boundaries next to a "0" cell tend to be shifted towards the center of the "0" cell. They seem to "wander" into the center of the "0" cell. At high recording densities peak shifting can cause errors in the processed read signal.
Peak shifting is characteristic of all magnetic disks and tapes, including single-density flexible disks, but can be more serious with modified frequency modulation (MFM) and modified-modified frequency modulation (M.sup.2 FM), see "Computer Design", supra.
Two methods are known to compensate for peak shifting. One of them influences the writing operation, where a write signal, consisting of electrical pulses and gaps, is applied to the write head. This method makes use of shifting those pulses, which are located adjacent to a gap, away from this gap ("pre-write compensation method"). The other compensation method influences the reading operation. It makes use of re-shifting read signal peaks, which are located adjacent to a gap, to their proper position ("post-read compensation method").
A read back circuit for a magnetic recording device is known from the brochures "OEM, Floppy Disk Drive FDD 100-8" and "OEM, Floppy Disk Drive FDD 200-8" by Siemens Corporation, Anaheim, California. In FIGS. 3.31 and 3.32, respectively, there is disclosed a read back circuitry for a floppy disk drive. The read back circuitry comprises a write/read head which reads data from a floppy disk during read operation. The output terminals of the head are connected via a field effect transistor (FET) in each connection line to a high-gain linear amplifier. The FETs are actuated during the read operation mode to pass read data from the head to the preamplifier. The output of the preamplifier is connected to the input of a filter network, the output of which is connected to a differentiating network. The filter network is a 3-pole linear-phase bandpass filter comprising a series and parallel combination of capacitors, resistors and chokes, see FIGS. 3.32 and 3.33, respectively. The differentiating network comprises an amplifier and a resonance circuit which is connected to the amplifier and which has connected in series a capacitor, an inductor and a resistor, see FIGS. 3.33 and 3.34 respectively. The differentiating network provides a 90-degree delay to convert the incoming read data signal peaks to distorted zero crossings. Zero crossings are detected by a crossover detector, which is connected to the output of the differentiating network. The crossover detector is formed by a comparator and a bidirectional one-shot. The comparator is driven by the analog output of the differentiator and provides a data pulse for each zero crossing. The floppy disk drive also comprises a counter which determines whether the write/read head is positioned on a track above or below a predetermined number. There is provided no read back compensates circuit which compensated for peak shifting.
It is known in magnetic recording that any recording device and in particulary any disk drive has a so-called "worst case" pattern for peak shifting. For a double-sensity encoding scheme (MFM) in a floppy disk drive, for instance, the "worst case" pattern would be 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, etc. (so-called 6DB6 pattern). An investigation of this pattern by a spectrum analyzer indicates that apart from the fundamental the pattern contains a second harmonic. Other "worst case" patterns reveal different harmonics. It is also well-known that, in order to push the shifted peaks back to their proper position, a certain harmonic should be added to the original signal. Such a read back compensation circuit is based on the post-read method.
In the field of magnetic recording it is also known to use Equalizers in the recording and/or reproducing operation, see for instance "Magnetic Recording" by Charles E. Lowman, McGraw Hill Book Co., New York, pages 86 to 89 and 147 to 149.